System with controller and memory

ABSTRACT

According to the system of the present invention, data (DQ) signals are outputted/received between a controller  100  and a memory  200  based on a data strobe signal sent out from the controller  100 . The data strobe signal is independently and completely separated from a clock signal. The data strobe signal has a frequency different from a clock signal. Therefore, the memory  200  is not required to generate a read data strobe signal from the clock signal nor to send the read data strobe signal in synchronization with the clock signal.

BACKGROUND OF THE INVENTION

This invention relates to a system having a controller and a memory and,more particularly, to a data sending/receiving operation between thecontroller and the memory.

In a conventional data sending/receiving operation between thecontroller and the memory, either the controller or the memory whoserves as a sender generates a data strobe signal which is synchronizedwith a clock signal. Data signals are sent out from the sender issynchronized with the data strobe signal as disclosed in U.S. Pat. No.6,789,209B1, Description of the Related Art, the contents of U.S. Pat.No. 6,789,209B1 being incorporated herein by reference in theirentireties.

U.S. Pat. No. 6,789,209B1 points out some problems on the use of thedata strobe signal and, alternative to the data strobe signal, proposesa system which sends/receives the data in synchronization with the clocksignal.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system based on anew protocol which is different from the above-described conventionaltechniques.

One aspect of the present invention provides a system comprising acontroller adapted to send out a first data strobe signal and a writedata signal in a write operation. The write data signal is synchronizedwith the first data strobe signal. The controller is adapted, in a readoperation, to send out a second data strobe signal and to receive a readdata signal in synchronization with a read data strobe (RDQS) signal.The read data strobe signal corresponds to the second data strobesignal. The system also comprises a memory adapted to receive the writedata signal in synchronization with the first data strobe signal in thewrite operation. The memory is adapted, in the read operation, to sendout the read data strobe signal in response to the second data strobesignal and to send out the read data signal synchronized with the readdata strobe signal.

The controller may be further adapted to produce a clock signal and tosend the clock signal to the memory. Each of the first and the seconddata strobe signals and the read data strobe signal may be independentof and separated from the clock signal.

Each of the first and the second data strobe signals and the read datastrobe signal may has a frequency different from the clock signal.

Each of the first and the second data strobe signals and the read datastrobe signal may not be in synchronization with the clock signal.

Another aspect of the present invention provides a system wherein thememory comprises a low speed memory and a high speed memory. Thecontroller sends out a clock signal having a clock frequency. Thecontroller also sends out first and second lower data strobe signals asthe first and the second data strobe signals for the low speed memoryand receives lower read data strobe signal as the read data strobesignal from the low speed memory. Each of the first and the second lowerdata strobe signals and the lower read data strobe signal has afrequency equal to or lower than the clock frequency. The controllersends out first and second higher data strobe signals as the first andthe second data strobe signals for the high speed memory and receiveshigher read data strobe signal as the read data strobe signal from thehigh speed memory. Each of the first and the second higher data strobesignals and the higher read data strobe signal has a frequency equal toor 2^(n) times (n being an integer) as the clock frequency.

An appreciation of the objectives of the present invention and a morecomplete understanding of its structure may be had by studying thefollowing description of the preferred embodiment and by referring tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing a structure of a systemaccording to a first embodiment of the present invention;

FIG. 2 is a schematic diagram showing a structure of a controller ofFIG. 1;

FIG. 3 is a schematic diagram showing a structure of a memory of FIG. 1;

FIG. 4 shows an example of a data I/O (input/output) synchronizationsignal generating section and a data I/O section of FIG. 3;

FIG. 5 shows another example of the data I/O synchronization signalgenerating section and the data I/O section of FIG. 3;

FIG. 6 is an example of a timing chart showing a write operation of thememory in the system of FIG. 1 (double data rate);

FIG. 7 is another example of a timing chart showing a write operation ofthe memory in the system of FIG. 1 (quad data rate);

FIG. 8 is an example of a timing chart showing a read operation of thememory in the system of FIG. 1 (double data rate);

FIG. 9 is another example of a timing chart showing a read operation ofthe memory in the system of FIG. 1 (quad data rate);

FIG. 10 is a schematic diagram showing an application of the systemaccording to the first embodiment of the present invention.

FIG. 11 is a schematic diagram showing a structure of a controllerapplicable to the system of FIG. 10;

FIG. 12 is a schematic diagram showing a structure of a controlleraccording to a second embodiment of the present invention;

FIG. 13 is an example of a timing chart showing a write operation of thememory in the system according to the second embodiment (double datarate);

FIG. 14 is another example of a timing chart showing a write operationof the memory in the system according to the second embodiment (quaddata rate);

FIG. 15 is an example of a timing chart showing a read operation of thememory in the system according to the second embodiment (double datarate); and

FIG. 16 is another example of a timing chart showing a read operation ofthe memory in the system according to the second embodiment (quad datarate).

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DESCRIPTION OF PREFERRED EMBODIMENTS 1st Embodiment

Referring to FIG. 1, a system according to a first embodiment of thepresent invention has a controller 100 and a memory 200. The controller100 outputs a clock signal, command signals, and address signals to thememory 200. The controller 100 also outputs and receives data (DQ)signals into/from the memory 200.

The controller 100 is adapted to send out a first data strobe signal anda write data signal in a write operation. The write data signal issynchronized with the first data strobe signal. The controller 100 isalso adapted, in a read operation, to send out a second data strobesignal and to receive a read data signal in synchronization with a readdata strobe (RDQS) signal. The RDQS signal corresponds to the seconddata strobe signal.

The memory 200 is adapted to receive the write data signal insynchronization with the first data strobe signal in the writeoperation. The memory 200 is adapted, in the read operation, to outputthe read data strobe signal in response to the second data strobe signaland to send out the read data signal synchronized with the read datastrobe signal.

In this embodiment, each of the first and the second data strobe signalsand the read data strobe signal is independent of and separated from theclock signal in detail, each of the first and the second data strobesignals and the read data strobe signal has a frequency different fromthe clock signal, and is not in synchronization with the clock signal inthis embodiment.

In the data read operation, the data strobe signal sent out from thecontroller 100 loopbacks via an interface section (not shown) of thememory 200 and is used as a data reception synchronizing signal at thecontroller 100.

In the system according to the first embodiment, the write/readoperations to/from the memory 200 are controlled by the data strobesignal outputted from the controller 100. The write and the readoperations can be carried out without the use of and independently fromthe clock signal. Therefore, it is not necessary for the memory 200 toproduce the data strobe signal from the clock signal nor to make theread data strobe signal synchronize with the clock signal.

Now, description will be made about the system in which the clock signalhas a constant and fixed frequency while the data strobe signal has thefrequency different from the clock signal.

Referring to FIG. 2, the controller 100 according to the firstembodiment has a C/A (command/address) control circuit 10, a PLL/DLL 20,a divider 30, a data strobe signal generating section 40, aninput/output control circuit 50, a data synchronous output section 60,and a data synchronous capture section 70.

Supplied with command/address outputs, the C/A control circuit 10generates internal command signals and internal address signals. Theinternal command signals and the internal address signals are sent tothe memory 200 via a buffer as the command signals and the addresssignals, respectively. In the present embodiment, the internal commandsignals are also sent to the I/O control circuit 50.

Supplied with a base clock signal, the PLL/DLL circuit 20 generates aninternal clock signal and a signal having the frequency at 2^(n) times(n being a positive integer) based on the base clock signal. The divider30 generates a signal having the frequency at 2^(p) times (p being anegative integer) as the internal clock signal. Each of the PLL/DLLcircuit 20 and the divider 30 functions as an intermediate clock signalgenerator for generating a plurality of intermediate clock signalsincluding the internal clock signals. Each of the plurality ofintermediate clock signals has different frequency. In the presentembodiment, the intermediate clock signal generator generates theintermediate clock signals each of which has the frequency at 2^(m)times (m being an arbitral integer) as the internal clock signal. Theinternal clock signal is sent to the memory 200 via the buffer as theclock signal. As shown in FIG. 1, the internal clock signal alsofunctions as an operation clock in the controller 100.

The data strobe signal generating section 40 receives the plurality ofintermediate clock signals from the PLL/DLL circuit 20 and the divider30 and selects one of the intermediate clock signals. Based on theselected intermediate clock signal, the data strobe signal generatingsection 40 generates an internal data strobe signal under the control ofthe input/output control circuit 50. The internal data strobe signal issent out as the data strobe signal to the memory 200 via the buffer. Inthe present embodiment, the internal data strobe signal is also sent tothe data synchronous output section 60.

The input/output control circuit 50 receives the internal command signaland, if the received command is a write command signal, activates thedata synchronous output section 60. If the received command is a readcommand signal, the input/output control circuit 50 activates the datasynchronous capture section 70. The input/output control circuit 50 ofthe present embodiment also outputs a frequency selecting signal to thedata strobe signal generating section 40. The frequency selecting signalindicates one of the intermediate clock signals to be selected for usein generating the internal data strobe signal. In the presentembodiment, the data strobe signal generating section 40 is a selectorwhich is brought into the activation state for the predetermined periodof time while the frequency selecting signals are supplied thereto. Theinput/output control circuit 50 determines the activation period of thedata strobe signal generating section 40, i.e., the period of producingthe frequency selecting signals. During the activated period, the datastrobe signal generating section 40 outputs the data strobe signalhaving the predetermined length in consideration of the preamble period.In addition, the input/output control circuit 50 determines a timing tostart the activation period of the data strobe signal generating section40 in consideration of a latency corresponding to the write/readcommands.

While being activated under the control of the input/output controlcircuit 50, the data synchronous output section 60 outputs the data asthe DQ signal to the memory 200 via the buffer in synchronization withthe internal data strobe signal.

On the other hand, while being activated under the control of theinput/output control circuit 50, the data synchronous capture section 70captures the read data, i.e., the DQ signal in synchronization with theRDQS signal which was sent out from the memory 200.

Referring to FIG. 3, the memory 200 according to the first embodimenthas at least a data I/O (input/output) section 210, a data I/Osynchronous signal generating section 220, and an internal clock signalgenerating section 230. The data I/O (input/output) section 210 sendsand receives the DQ signals between the controller 100. The data I/Osynchronous signal generating section 220 generates a synchronous signalfor controlling the operation of the data I/O section based on the datastrobe signal and generates the RDQS signal from the data strobe signalin the read operation. The internal clock signal generating section 230generates an internal clock signal upon receiving the clock signal andsends out the internal clock signal to circuits for command/address andso on. As shown in FIG. 3, the input/output operation of the memory 200is carried out based on the data strobe signal received from thecontroller 100 and is completely separated from the clock signal.

FIG. 4 shows the data I/O section 210 and the data I/O synchronoussignal generating section 220 a applicable to the embodiment of thepresent invention. The data I/O synchronous signal generating section220 a has the buffer for generating the input synchronous signal fromthe received data strobe signal and an output synchronous signalgenerating section for generating the RDQS signal as well as the outputsynchronous signal from the intermediate synchronous signal from thebuffer. The data I/O section 210 has a data latch for latching the DQsignal in synchronization with the input synchronous signal andoutputting the same as the write data, and a data output buffer forreceiving the read data and outputting the same as the DQ signal insynchronization with the output synchronous signal to the controller100. In FIG. 4, the data strobe signal and the RDQS signal aretransmitted in a single-ended transmission system.

FIG. 5 shows the data input/output (I/O) section 210 and the data I/Osynchronous signal generating section 220 b applicable to the embodimentof the present invention. According to the example shown in FIG. 5, thedata strobe signal and the RDQS signal are transmitted in thedifferential transmission system. The data I/O synchronous signalgenerating section 220 b has the structure same as the that describedabove with reference to FIG. 4 except that the structure of the receiveramplifier and the driver are different.

FIGS. 6 to 9 are timing charts showing the operations of the systemaccording to the embodiment of the present invention. FIGS. 6 and 7 arethe timing charts showing the write operation of the memory 200 andFIGS. 8 and 9 are the timing charts showing the read operation of thememory 200.

Referring to FIG. 6, the clock signal and the data strobe signal has thesame frequencies. Data are captured in synchronization with the risingedge and the falling edge of the data strobe signal. In the exampleshown in FIG. 6, the data rate of the data strobe signal is twice as thefrequency of the clock signal.

Referring to FIG. 7, the data strobe signal has the frequency twice asthe frequency of the clock signal. Data are captured in synchronizationwith the rising edge and the falling edge of the clock signal. In thiscase, the data rate of the data strobe signal is four times as thefrequency of the clock signal. Referring to FIG. 2 together with FIG. 7,the example shown in FIG. 7 can be obtained by generating the datastrobe signal based on the intermediate clock signal having thefrequency twice as that of the internal clock signal selected by thedata strobe signal generating section 40.

Referring to FIG. 8, the clock signal and the data strobe signal has thesame frequencies. Data are sent out in synchronization with the risingedge and the falling edge of the data strobe signals. The data rate istwice as the frequency of the clock signal.

Referring to FIG. 9, the data are captured in synchronization with therising edge and the falling edge of the frequency of the data strobesignal having the frequency twice as that of the clock signal. The datarate is four times as the frequency of the clock signal. Referring toFIG. 2 together with FIG. 9, the example shown in FIG. 9 can be obtainedby selecting one of the intermediate clock signals having the frequencytwice as that of the internal clock signal and generating the internaldata strobe signal based on the selected one of the intermediate clocksignals by the data strobe signal generating section 40.

Referring to FIGS. 8 and 9, tRDQSD represents the spontaneous delayamount which is added to the data strobe signal while the data strobesignal is outputted as the RDQS signal via the data I/O synchronoussignal generating section 220 a (FIG. 4) or 220 b (FIG. 5). The delayamount shown in FIGS. 8 and 9 is not a strict delay amount such as thedelay replica incorporated in the DLL. In FIGS. 8 and 9, the depictionof the tRDQSD helps it clearly be understood that the DQ signal issynchronized with the RDQS signal but is not synchronized with the clocksignals. According to the present embodiment, the DQ signal can betransmitted/received in synchronization with only the data strobesignal. Therefore, the structure of the input/output section of thememory 200 can be simplified.

Next, description will be made about another example of the firstembodiment. Referring to FIG. 10, the system has a high speed memory anda low speed memory. Hereinafter the high speed memory is called a firstmemory 300 and the low speed memory is called a second memory 400. Thefirst memory 300 has the high data rate while the second memory 400 hasthe low data rate.

According to another example, the controller 100 sends out the clocksignal having the clock frequency. The controller 100 also sends outfirst and second lower data strobe signals as the first and the seconddata strobe signals for the low speed memory 400 and receives lower readdata strobe signal as the read data strobe signal from the low speedmemory 400. Each of the first and the second lower data strobe signalsand the lower read data strobe signal has a frequency equal to or lowerthan the clock frequency.

The controller 100 sends out first and second higher data strobe signalsas the first and the second data strobe signals for the high speedmemory 300 and receives higher read data strobe signal as the read datastrobe signal from the high speed memory 300. Each of the first and thesecond higher data strobe signals and the higher read data strobe signalhas a frequency equal to or 2^(n) times (n being an integer) as theclock frequency.

According to the first embodiment, the frequency of the clock signal isfixed and constant while the frequency of the data strobe signal isvariable and different from the clock signal. Therefore, both the highspeed memory 300 and the low speed memory 400 can be operated by thedata strobe signals each having the suitable data rate for each memory.

FIG. 11 shows an example of the controller 100 capable of automaticallycontrolling the frequency of the data strobe signal when the firstmemory 300 and the second memory 400 are distinctly different from eachother within an address space. In this case, the frequency of the datastrobe signal is automatically controlled depending on the content ofthe address. In FIG. 11, the same reference numbers are given to thestructures same as those illustrated in FIG. 10 and the descriptiontherefor will be omitted.

Referring to FIG. 11, the internal address signal outputted from the C/Acontrol circuit is supplied to the input/output control circuit 52. Theinput/output control circuit 52 receives the internal address signal andjudges whether the received signal is designated for the first memory300 or the second memory 400. Based on the judgment result, theinput/output control circuit 52 sends out the frequency selection signalto the data strobe signal generating section 40. The frequency selectionsignal is used to select the frequency corresponding to the designatedmemory. According to the first embodiment, the data strobe signalgenerating section 40 sends out the data strobe signal having thefrequency suitable for either the first memory 300 or the second memory400 under the control of the input/output control circuit 52.

Second Embodiment

The system according to the second embodiment of the present inventionis a modified example of the first embodiment of the present invention.According to the second embodiment, the frequency of the data strobesignal is fixed and constant while the frequency of the clock signal isvariable. The memory 200 of the second embodiment has the same structureas the first embodiment. The controller 100 of the second embodiment hasthe structure slightly different from the first embodiment. Hereinbelow,the detailed description will be made about the different parts of thestructure with reference to FIGS. 12 to 16. In the second embodiment,the same reference numbers are given for the parts those are same as thefirst embodiment and the description therefor will be omitted.

Referring to FIG. 12, the controller according to the second embodimentof the present invention has the C/A control circuit 10, the PLL/DLL 20,the divider 30, a clock signal selecting section 42, the data strobesignal generating section 44, the input/output (I/O) control circuit 54,the data synchronous output section 60, and the data synchronous capturesection 70.

Under the control of the I/O control circuit 54, the clock signalselecting section 42 selects one of the plurality of intermediate clocksignals outputted from the PLL/DLL 20 and the divider 30. The clocksignal selecting section 42 outputs the selected intermediate clocksignal as the clock signal to the memory 200 via the buffer.

The data strobe signal generating section 44 receives a specific one ofthe plurality of intermediate clock signals outputted from the PLL/DLL20 and the divider 30. Under the control of the I/O control circuit 54,the data strobe signal generating section 44 generates the internal datastrobe signal based on the received intermediate clock signal and sendsout the same as the data strobe signal to the memory 200 via the buffer.

The I/O control circuit 54 outputs the frequency selecting signal to theclock signal selecting section 42. The frequency selecting signal isindicative of the signal to be selected among the intermediate clocksignals. The I/O control circuit 54 also outputs an activation signal tothe data strobe signal generating section 44. The activation signalactivates the data strobe signal generating section 44 for a certain andsuitable period of time in consideration of the latency corresponding tothe write/read command. The input/output control circuit 54 controls thedata synchronous output section 60 and the data synchronous capturesection 70 in the manner same as the first embodiment.

FIGS. 13 to 16 show the operation timing charts of the memory 200 of thesystem according to the present embodiment.

FIGS. 13 and 14 show the timing charts of the data write operation.Referring to FIG. 13, the frequency of the clock signal and the datastrobe signal are the same. The data rate of the data strobe signal istwice as the frequency of the clock signal. Referring to FIG. 14, thefrequency of the clock signal is ½ times as the frequency of the datastrobe signal. The data rate of the data strobe signal is four times asthe frequency of the clock signal.

FIGS. 15 and 16 show the timing charts of the data read operation.Referring to FIG. 15, the frequency of the clock signal and the datastrobe signal are the same. The data rate of the data strobe signal istwice as the frequency of the clock signal. Referring to FIG. 16, thefrequency of the clock signal is ½ times as the frequency of the datastrobe signal. The data rate of the data strobe signal is four times asthe frequency of the clock signal.

The present application is based on Japanese patent applications ofJP2006-160204 filed before the Japan Patent Office on Jun. 8, 2006, thecontents of which are incorporated herein by reference.

While there has been described what is believed to be the preferredembodiment of the invention, those skilled in the art will recognizethat other and further modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended to claimall such embodiments that fall within the true scope of the invention.

1. A system comprising: a controller adapted to send out a first datastrobe signal and a write data signal in a write operation, the writedata signal being synchronized with the first data strobe signal, thecontroller being adapted, in a read operation, to send out a second datastrobe signal and to receive a read data signal in synchronization witha read data strobe signal, the read data strobe signal being received bythe controller in response to the second data strobe signal; and amemory adapted to receive the write data signal in synchronization withthe first data strobe signal in the write operation, the memory beingadapted, in the read operation, to output the read data strobe signal inresponse to the second data strobe signal and to send the read datasignal synchronized with the read data strobe signal.
 2. The systemaccording to claim 1, wherein the controller is further adapted toproduce a clock signal and to send the clock signal to the memory, eachof the first and the second data strobe signals and the read data strobesignal being independent of and separated from the clock signal.
 3. Thesystem according to claim 2, wherein each of the first and the seconddata strobe signals and the read data strobe signal has a frequencydifferent from the clock signal.
 4. The system according to claim 2,wherein each of the first and the second data strobe signals and theread data strobe signal is not in synchronization with the clock signal.5. A system comprising: a controller adapted to send out a first datastrobe signal and a write data signal in a write operation, the writedata signal being synchronized with the first data strobe signal, thecontroller being adapted, in a read operation, to send out a second datastrobe signal and to receive a read data signal in synchronization witha read data strobe signal, the read data strobe signal being received inresponse to the second data strobe signal; and a memory adapted toreceive the write data signal in synchronization with the first datastrobe signal in the write operation, the memory being adapted, in theread operation, to output the read data strobe signal in response to thesecond data strobe signal and to send the read data signal synchronizedwith the read data strobe signal, wherein the controller is furtheradapted to produce a clock signal and to send the clock signal to thememory, each of the first and the second data strobe signals and theread data strobe signal being independent of and separated from theclock signal, the memory comprises a low speed memory and a high speedmemory, the clock signal has a clock frequency, the controller sends outfirst and second lower data strobe signals as the first and the seconddata strobe signals for the low speed memory and receives lower readdata strobe signal as the read data strobe signal from the low speedmemory, each of the first and the second lower data strobe signals andthe lower read data strobe signal having a frequency equal to or lowerthan the clock frequency, and the controller sends out first and secondhigher data strobe signals as the first and the second data strobesignals for the high speed memory and receives higher read data strobesignal as the read data strobe signal from the high speed memory, eachof the first and the second higher data strobe signals and the higherread data strobe signal having a frequency equal to or 2^(n) times (nbeing an integer) as the clock frequency.
 6. A controller for use in asystem, the system including a memory, the controller being adapted tosend out a first data strobe signal and a write data signal to thememory in a write operation, the write data signal being synchronizedwith the first data strobe signal, the controller being adapted to sendout a second data strobe signal to the memory and to receive a read datasignal in synchronization with a read data strobe signal from the memoryin a read operation, the read data strobe signal received by thecontroller corresponding to the second data strobe signal in the readoperation.
 7. The controller according to claim 6, wherein thecontroller is further adapted to produce a clock signal and to send theclock signal to the memory, each of the first and the second data strobesignals and the read data strobe signal being independent of andseparated from the clock signal.
 8. The controller according to claim 7,wherein each of the first and the second data strobe signals and theread data strobe signal has a frequency different from the clock signal.9. The controller according to claim 7, wherein each of the first andthe second data strobe signals and the read data strobe signal is not insynchronization with the clock signal.
 10. A memory for use in a system,the system including a controller, the controller outputting a datastrobe signal in a read operation, the memory being adapted to send outa read data strobe signal to the controller in response to the datastrobe signal and to send out a read data signal synchronized with theread data strobe signal in a read operation.
 11. The memory according toclaim 10, the controller being further adapted to send out a clocksignal and to send the clock signal to the memory, wherein each of thedata strobe signal and the read data strobe signal is independent of andseparated from the clock signal.
 12. The memory according to claim 11,wherein each of the data strobe signal and the read data strobe signalhas a frequency different from the clock signal.
 13. The memoryaccording to claim 11, wherein each of the data strobe signal and theread data strobe signal is not in synchronization with the clock signal.14. The system of claim 1, wherein in the read operation the memoryretransmits the second data strobe signal as the read data strobesignal.
 15. The system of claim 14, wherein in the read operation thememory retransmits the second data strobe signal with a time delay asthe read data strobe signal.
 16. The controller of claim 6, wherein inthe read operation the controller receives a read data signal from thememory which is a retransmission of the second data strobe signal. 17.The controller of claim 16, wherein in the read operation the controllerreceives a read data signal from the memory which is a retransmission ofthe second data strobe signal with a time delay.
 18. The memory of claim10, wherein in the read operation the read data strobe signal is aretransmission of the received data strobe signal.
 19. The memory ofclaim 18, wherein in the read operation the read data strobe signal is aretransmission of the received data strobe signal with a time delay. 20.A system comprising: a first memory operating at a high data rate; asecond memory operating at a low data rate; a controller connected tothe first and second memories, wherein the controller provides firstdata strobe signals having a frequency higher than a system clockfrequency to operate the first memory at the high data rate, andprovides second data strobe signals having a frequency lower than thesystem clock frequency to operate the second memory and a low data rate.21. The system according to claim 20, wherein the first and secondmemories transfer data synchronously with the first and second datastrobe signals, respectively, having frequencies different than thesystem clock frequency.
 22. The system according to claim 20, whereinthe first memory and the second memory are different from each otherwithin an address space.
 23. The system according to claim 22, whereinthe controller comprises a control circuit which determines whether asignal is designated for the first memory or the second memory.
 24. Thesystem according to claim 23, wherein the frequency of the first andsecond data strobe signals is automatically controlled based on thedetermination of the first and second memories made by the controlcircuit.
 25. The system according to claim 1, further comprising firstand second data strobe lines each provided between the controller andthe memory, the first data strobe line transferring the first datastrobe signal in the write operation from the controller to the memory,the first data strobe line transferring the second data strobe signal inthe read operation from the controller to the memory, and the seconddata strobe line transferring the read data strobe signal in the readoperation from the memory to the controller.
 26. A controller comprisinga first terminal, a second terminal, and a third terminal, thecontroller being adapted to produce a first data strobe signal at thefirst terminal and a write data signal at the third terminal in a writeoperation, the write data signal being synchronized with the first datastrobe signal, the controller being adapted to produce a second datastrobe signal at the first terminal and to receive a read data signal atthe third terminal in synchronization with a read data strobe signalarriving at the second terminal from outside of the controller in a readoperation.
 27. The controller according to claim 26, wherein the readdata strobe signal is generated outside of the controller in response tothe second data strobe signal.